TOF SIMS characterization of SEI layer on battery electrodes

In the last decade, many studies applied surface analysis techniques (SEM, XPS and SIMS) to understand the formation of SEI layers on Li-ion battery electrodes. This work was meant as a comparative model study of the SEI layer formation, which combined in situ SEM imaging with TOF SIMS depth profiling of four samples of the same graphite electrode material, which was subjected to different charge-discharge cycling schemes in a Li-ion battery. Besides comparing compositions of sub-surface regions of these differently processed electrodes, we wanted to know whether these compositions depend on after-cycling sample preparation, in particular if a brief exposure of these samples to air would affect the compositions measured by TOF SIMS. We found that the exposure to air (1) increases secondary ion yield for all species, and (2) changes shapes of SIMS depth profiles for some key species. For selected samples, we also conducted a comparison between the conventional single beam TOF-SIMS depth profiling and a high resolution dual beam depth profiling and found that the former approach can detect the same features in depth profiles as the latter one. We interpreted this as an indication that the sample surface morphology (high roughness) could be a limiting factor in this SEI study, suggesting that specially designed model samples with smooth surfaces are a better choice for future studies.